Date of Award

8-9-2014

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Mohammod Ali

Abstract

Sensors and sensing system are playing dominant roles in monitoring the health of infrastructure, such as bridges, power lines, gas pipelines, rail roads etc. Sensing modalities employing Surface Acoustic Waves (SAW), Electromagnetic (EM) and optical have been investigated and reported. Sensors that utilize the perturbation of EM fields as function of the change in the physical structural or material phenomenon are of particular interest because of their inherent synergy with electronic system and diagnostic techniques, e.g. Time Domain Reflectometry (TDR), Joint-Time-Frequency-Domain-Reflectometry (JTFDR). The focus of this work is to study and develop new sensing and monitoring concepts that are based on EM principles.

First, the analyses, design and development of a static electric field type sensor are presented for application in embedded concrete moisture content measurement. The analytical formulation and results based on conformal mapping method for an interdigitated sensor clearly show the dependency of the field penetration depth and the inter-electrode capacitance on the electrode sizes and their spacings. It is observed that larger electrode size and small separation are needed in order to achieve substantially higher capacitance or large field penetration depth. A meander and a circular sensor are fabricated and tested to demonstrate concrete moisture content measurements that show that moisture content is a linear function of sensor interelectrode capacitance. Second, sub-wavelength dimension non-intrusive wave launchers are designed and tested that can launch TDR or JTFDR type broadband surface wave waveforms in the VHF-UHF bands in order to detect cable faults. Greater than 3:1 transmission bandwidth (100-300 MHz) is obtained with a cylindrical launcher on square orthogonal ground plane while with a CSW launcher more than an octave (100-240 MHz) bandwidth is achieved. Open circuit faults are detected using surface waves and TDR on two XLPE cables. Third, a new mathematical method is developed that can be used to determine the changes in the dielectric constant of a cable insulating material. By comparing the experimental JTFDR waveform signatures from a new and an aged cable, it is demonstrated that the change in the average dielectric constant of the insulation material can be estimated from the phase transfer functions obtained from the FFT of measured magnitude and phase responses. The experimental data obtained for two types of cables, XLPE and EPR show that the dielectric constant decreases with accelerated aging. Finally, JTFDR surface wave sensing method is developed and applied to determine the locations of aging related insulation damage in power cables. The comparative power spectral responses of conducted and non-intrusive surface wave JTFDR waveforms clearly show the resulting bandwidth reduction in the latter primarily because of the reflective nature of the coupling. It is demonstrated that with the help of a non-intrusive wave launcher and a 120 MHz Gaussian chirp waveform the location of aging related insulation damages can be detected. Experiments conducted show the cross-correlation peaks at subsequent aging intervals as the cable is aged inside a heat chamber.

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